1. Technical Field
The present disclosure relates to imaging devices. More particularly, it relates to a circuit and method for measuring toner or ink levels in the imaging unit of an imaging device.
2. Description of the Related Art
Image forming devices such as copiers, laser printers, facsimile machines and the like typically use one or more toner containers to hold toner supply used for image forming processes. In some image forming devices, a large toner supply is provided in a reservoir in a toner cartridge that mates with a separate imaging unit. The imaging unit may include a developer unit having a sump that holds a smaller amount of toner, enough to ensure toner is adequately supplied by a toner adder roll and a developer roll, both of which are located in the developer unit, to a photoconductive drum. As toner within the imaging unit sump is depleted due to printing operations, additional toner is transferred from the toner cartridge to the imaging unit sump.
To ensure satisfactory operation of the imaging unit to transfer toner, the toner level within the imaging unit sump is maintained at a proper level. For example, if the imaging unit sump holds too much toner, toner may pack in the imaging unit sump, leak out of the ports and eventually break other components located inside and outside the imaging unit. If the toner level in the imaging unit sump gets too low, the toner adder roll may starve, causing a doctor blade of the imaging unit to film and damage the developer roll which may eventually impair the future performance of the imaging unit. As such, it is desirable to know the toner level in the imaging unit sump so as to effectively determine when to move toner from toner cartridge to the imaging unit sump.
Some methods for determining toner level in a container use estimates of toner use and accumulation based on print or time counts. However, these methods may not be accurate due to variability in factors such as the environment, developer roll age, toner patch sensing cycles, and toner transfer parameters.
Other known techniques for sensing or determining toner level include the use of electrical sensors that measure the motive force required to drive an agitator within a toner container, optical devices including mirrors and toner dust wipers in a container, and other opto-electromechanical devices such as a flag that moves with the toner level to actuate a sensor which triggers only when the toner volume reaches a predetermined level. Unfortunately, the addition of moving hardware increases component complexity and opportunities for errors. For instance, toner agitation may create unwanted toner dust in addition to the added complication of moving hardware.
Other techniques for sensing or determining toner level include use of a capacitive sensor disposed within a toner container, such as a waste toner container, and circuitry for sensing the capacitance of the capacitive sensor as toner levels in the container change. In one existing implementation, illustrated in FIG. 1, an AC signal generator 101 is connected to the capacitor Cx, representing the capacitive sensor to be measured, and applies a generally square wave signal thereto. Capacitor Cx couples the AC signal generator 101 to a high-pass amplifier 102 which buffers and amplifies the AC square wave signal. A synchronous rectifier 103, which is coupled to the output of the high pass amplifier 102, operates at the same frequency as AC signal generator and is synchronized thereto. The synchronous rectifier 103 converts the square-wave (bipolar) signal into a unipolar signal. A low-pass filter 104 receives the unipolar signal from the synchronous rectifier 103 and amplifies and smoothes the unipolar signal. The low pass filter 104 outputs a DC output voltage Vout.
Output voltage Vout is modulated by a modulator circuit 105 to a square wave at the frequency of the AC signal generator 101 and synchronized thereto. The output of modulator circuit 105 is fed through a reference capacitor Cref back to the input of high pass amplifier 102. The modulator circuit 105 inverts the phase of the signal from the AC signal generator 101 so that the modulator circuit 105 and the AC signal generator 101 are 180 degrees out of phase with each other. The feedback loop controls output voltage Vout such that the input to high pass amplifier 102 is effectively a DC signal. In other words, the AC current through capacitor Cx is substantially balanced by the current through reference capacitor Cref. The transfer function for this circuit isVout=VAC*Cx/Cref,where VAC is the voltage output of the AC signal generator 101. With Vout, VAC and Cref being known values, the capacitance of capacitor Cx can be determined which is indicative of the amount of toner existing in the toner container in which capacitor Cx is disposed. The circuit of FIG. 1 may be scaled to measure capacitors between about 0 pF and about 22 pF.
The absolute accuracy of the CTLS circuit is of importance in order to obtain accurate toner level measurements. If the CTLS circuit exceeds its error budget, the developer unit may either overfill or underfill. Overfill can cause the fill auger of the developer unit to break as it packs toner into the developer unit. Underfill can cause the developer unit to run dry which causes the doctor blade of the developer unit to undesirably strip a film off the developer roll. The error budget for the CTLS circuit of FIG. 1 may be less than 1.5 pF. Known error sources include: 1) a power supply that varies over time and temperature; 2) analog-to-digital circuitry characteristics (reference voltage, offset, non-linearity and gain), at least some of which may vary over time and temperature; 3) reference capacitor tolerance; 4) attenuation resistor tolerance; 5) operational amplifier offset voltage variation over time and temperature.
It is difficult to maintain the desired accuracy of the CTLS circuit with the above set of error sources. What is needed is a CTLS circuit with well controlled errors so the desired accuracy may be maintained.